The invention disclosed and claimed herein generally pertains to magnetic resonance (MR) angiography, i.e., to MR imaging of an artery or like vessel carrying blood or other fluid. More particularly, the invention pertains to a method of the above type wherein MR data are acquired at each of a number of scan locations or stations, which are spaced along a vessel of comparatively great length. Even more particularly, the invention pertains to a method of the above type wherein an amount of contrast agent, or bolus, moves along the vessel or other conduit, from station-to-station, and measures are taken to ensure that MR data is acquired at a particular station only or substantially when the bolus is located there.
It is now a well known practice in MR angiography to insert a volume of contrast agent, such as gadolinium chelate, into blood flowing along a vessel. The volume or mass of contrast agent is referred to as a bolus, and has the effect of shortening the T.sub.1 time of the blood. Thus, an MR image of the blood, acquired by a fast gradient echo or like technique, will show up very well with respect to adjacent stationary tissue of the vessel structure.
It is also well known that certain clinical assessments require imaging a vascular territory of comparatively great length. Using MR for these evaluations, therefore, necessitates the acquisition of MR data over several stations or scan locations, which are located at intervals along the vessel path of flow. To acquire data at a particular station, the patient is selectively positioned with respect to an MR scanner, typically by movement of a table supporting the patient. Data are then acquired from a series of slices taken through a region or section of the patient, which comprises the particular scan location or station. Thereafter, the patient is shifted, relative to the scanner, so that data may be acquired from another section of the patient, comprising another scan location or station. MR angiography employing this procedure in conjunction with an injection of a contrast bolus may be referred to as bolus chasing peripheral MR angiography.
At present, when a contrast agent is used in connection with a peripheral MR angiography exam, the first scan station is selected to be the section of the patient, along a vessel of interest, at which the bolus arrives first. When the scan at the first station is completed, the acquisition normally moves to the next scan station. However, the most appropriate time to move to the next station is not precisely known. For example, in the case of slow blood flow, the distal vasculature at the next scan station may not have had adequate time to fill with contrast material. On the other hand, if flow rate is greater than anticipated, the contrast agent may tend to move into stationary tissue adjacent to the next scan station, before data acquisition commences. In either case, contrast between moving fluid and stationary tissue may be significantly reduced at the next scan station. Moreover, undesirable effects, resulting either from flow rate which is too slow or too great, may tend to become progressively worse as imaging proceeds to subsequent scan stations, and as the total number of scan stations increases.